1. Field of the Invention
This invention relates to an adsorbent for removing sulfur compounds, which is used to remove sulfur compounds by adsorption from a fuel gas such as a city gas, liquefied petroleum gas or a natural gas. The invention also relates to a method for removing sulfur compounds from a sulfur compound-containing fuel gas by use of an adsorbent for removal of such sulfur compounds.
2. Prior Art
Lower hydrocarbon gases such as methane, ethane, propane, butane and the like, or gases such as a natural gas, a city gas, an LP gas or the like gas containing these hydrocarbon gases, are used not only as an industrial or domestic fuel, but also as a starting material for preparing hydrogen that is utilized as a fuel for fuel cells or an atmospheric gas. In a steam reforming process, which is an industrial preparation process of hydrogen, these lower hydrocarbon gases are reformed by the addition of steam in the presence of a catalyst, such as a Ni-based catalyst, a Ru-based catalyst or the like, thereby forming a reformed gas mainly composed of hydrogen.
A fuel gas such as a city gas, an LP gas or the like is usually incorporated with a sulfur compound, such as a sulfide, a thiophene or a mercaptan, for use as an odorant for the purpose of security against the leakage thereof. More particularly, sulfides include dimethyl sulfide (hereinafter referred to simply as DMS), ethylmethyl sulfide, diethyl sulfide and the like, thiophenes include tetrahydrothiophene (hereinafter referred to simply as THT), and mercaptans include t-butyl mercaptan (hereinafter referred to simply as TBM), isopropyl mercaptan, n-propyl mercaptan, t-amyl mercaptan, t-heptyl mercaptan, methyl mercaptan, ethyl mercaptan and the like.
DMS, THT and TBM are, in most cases, used as an ordinarily added odorant, and are usually added not only singly, but also in admixtures of two or more (e.g. both DMS and TBM have been added to almost all city gases in the Metropolitan area of Japan at present). The concentration of the odorant is at a level of several ppm without exception. The catalyst used for such a steam reforming process as set out above is poisoned with these sulfur compounds, thus leading to the degradation of its performance. Accordingly, these sulfur compounds in a fuel gas should be preliminarily removed from a fuel gas. Even if it is inevitable that a residual sulfur compound be contained in small amounts in the fuel gas from which the sulfur compounds have been removed, the amount of the residual sulfur compound should preferably be as small in concentration as possible.
For the removal of a sulfur compound from a fuel gas, it is usual to use a hydrodesulfurization process or a process using an adsorbent. The hydrodesulfurization process comprises adding hydrogen to a fuel gas, decomposing and converting a sulfur compound into hydrogen sulfide in the presence of a catalyst such as a Co—Mo catalyst, and desulfurizing by adsorption of hydrogen sulfide, which is a decomposition product, by means of a desulfurizing agent such as zinc oxide, iron oxide or the like. Although the hydrodesulfurization process is a reliable process, it is necessary to convert all sulfur compounds into hydrogen sulfide by hydrogenation and heating to about 300 to 400° C. In addition, since zinc oxide or iron oxide is used for adsorption and removal, operations become complicated. Accordingly, this process has been employed in a large-scale plant, but is difficult to apply to a small-sized apparatus.
On the other hand, the process using an adsorbent is one wherein a fuel gas is passed through an adsorbent mainly composed of activated carbon, a metal oxide, zeolite or the like to remove a sulfur compound by adsorption. Although the process using an adsorbent includes a process wherein adsorptivity is increased by application of heat, adsorption at normal temperatures is preferred because a simpler system is realized. A process of removing a sulfur compound at normal temperatures by use of an adsorbent needs neither heat or hydrogen as in a hydrodesulfurization process or a thermal adsorption process, and thus, is a simple desulfurization process.
As a matter of course, however, the process of removing a sulfur compound by use of an adsorbent is unable to remove the sulfur compound from a gas if the adsorbent is saturated with once-adsorbed sulfur compounds. Thus, an exchange or regeneration of an adsorbent is necessary. Because the required amount of an adsorbent and frequency of exchange are greatly influenced depending on the adsorptivity of an adsorbent, there is a demand for an adsorbent having a higher adsorptivity. The performance of an adsorbent is influenced, especially, by the properties of a sulfur compound. Hence, with a gas containing a plurality of sulfur compounds such as a city gas, for example, a single adsorbent should have a high adsorptivity for plural sulfur compounds. Otherwise, a very burdensome problem will arise, e.g. a plurality of adsorbents corresponding to individual sulfur compounds are undesirably required.
Up to now, various types of adsorbents for sulfur compounds in a gas have been proposed. For instance, in Japanese Laid-open Patent Application No. Hei 6-306377, mercaptans that are used as an odorant for fuel gases such as a city gas, an LP gas and the like are selectively removed in an oxygen-free atmosphere by means of a zeolite exchanged with a polyvalent metal other than hydrogen and/or an alkaline earth metal. It is stated that as the polyvalent metal ions, there are preferably used those ions of Mn, Fe, Co, Ni, Cu, Sn and Zn. The sulfur compounds, which are to be adsorbed according to this technique, are directed only to mercaptans that are easily adsorbed.
We conducted experiments using a number of commercially available adsorbents including various types of porous materials such as zeolites, activated carbon, metal compounds, activated alumina, silica gels, activated clays, clay minerals and the like. Part of the results is shown in Table 2 appearing hereinafter. As a result, it was found that a specific type of activated carbon and a specific type of zeolite (Japan Laid-open Patent Application No. Hei 10-237473) are effective for adsorption of sulfur compounds in fuel gases.
By the way, some fuel gases may contain a trace of moisture in the course of a manufacture process or a supply process. Especially, where a fuel gas containing moisture is treated with zeolite, it selectively adsorbs the moisture, so that the adsorptivity of a sulfur compound significantly lowers over the case where no moisture is contained or a very small amount of moisture is present. This is assumed for the reason that zeolite per se, which is utilized as a moisture absorber, is hydrophilic in nature and preferentially adsorbs moisture made of polar molecules. In view of this, the adsorbent for removal of sulfur compounds should selectively adsorb sulfur compounds alone in a fuel gas, and should also adsorb sulfur compounds selectively irrespective of the presence or absence of moisture in a fuel gas. However, prior-art adsorbents including those adsorbents set out in the above-discussed patent publications are not taken into consideration with regard to the selective adsorption.
As stated above, the amount of a residual sulfur compound contained in a fuel gas, from which sulfur compounds have been removed, should be at a concentration as low as possible when the fuel gas is used for steam reforming. This is for the purpose of preventing a steam reforming catalyst from poisoning with sulfur. Up to now, a copper-zinc adsorbent (Japanese Laid-open Patent Application No. Hei 6-256779) has been reported for use as an adsorbent for removing sulfur compounds from a gas to a very low concentration. However, this adsorbent has to be heated to a temperature of 150 to 250° C. in order to impart satisfactory properties thereto.
The assignee of the instant application previously proposed (Japanese Laid-open Patent Application No. Hei 10-237473) an adsorbent for sulfur compounds in gases, which is made of an Na—X type zeolite having a pore size of at least 5 angstroms or over. This adsorbent has excellent adsorptivity at normal temperatures. Although this adsorbent shows a satisfactory performance on gases having a low dew point, i.e. gases containing no or little moisture, however, it takes precedence of adsorption of moisture in a gas having a high dew point, so that the adsorptivity of sulfur compounds considerably lowers.
FIG. 1 is a graph showing measurements of the adsorbent. The test device and conditions are such that there is used a device as in Adsorption Test 1 described hereinafter wherein the adsorption test is carried out under different dew point conditions. As shown in FIG. 1, the adsorption performance is so excellent that the adsorption of sulfur is at 3 wt % at a dew point of −70° C. However, as the dew point increases or as the moisture content in the gas increases, the adsorption of sulfur sharply drops. For instance, the sulfur content at a dew point of −50° C. is at 1.5 wt %, that is about half at a dew point of −70° C. for the same adsorbent, and is as low as 0.2 wt % at a dew point of −30° C.